1. Field of the Invention
The invention relates to a mechanism for providing reactionless drive to a rotary member of, for example, an articulating probe head which may be mounted on the movable arm of a coordinate positioning machine.
2. Description of Related Art
In our U.S. Pat. No. 5,189,806 there is described an articulating probe head capable of orienting a probe with two degrees of freedom to enable the probe to be used in an operation for scanning the surfaces of workpieces.
In general, such a probe head includes a rotary drive mechanism having a relatively fixed supporting structure and a rotary member which is rotatable by a motor relative to the supporting structure about an axis of the structure. The motor includes, in the case of an electric motor for example, a stator operably connected to the supporting structure and a rotor operably connected to the rotary member. Torque generated by the motor and applied to the rotary member also causes an equal and opposite reaction torque to be applied to the stator and thus to the supporting structure.
This reaction torque can cause a rotation on the movable arm of the coordinate positioning machine on which the probe head is mounted, leading to errors in the measurements made by the machine.
In the specific example shown in the patent specification referred to, a second rotary drive mechanism is mounted on the output shaft of the first mechanism and rotates a second rotary member about an axis at right angles to the axis of the first rotary mechanism.
Because the articulating probe head described in the above patent specification is required at times to drive a probe mounted on it with an oscillatory motion, in one of the preferred embodiments, the so-called stator is mounted on a bearing to allow it to rotate freely relative to the support structure and the rotor, so that the angular momentum of the rotor is counter-balanced by the freely spinning stator.
Thus, when the motor is operated to accelerate the rotary member, the free spinning stator accelerates in the opposite direction reacting to the output rotor torque. Thus there is no natural path back to the static structure to react the rotor output torque, and thus no resultant torque is passed back to the machine on which the head is mounted.
However, there are occasions when it is necessary to control the speed of the rotatable stator. For example, there are opposing torques applied to the spinning stator which create losses in the angular momentum of the stator and which would tend to slow the stator down in a constant angular velocity rotor move. These losses are due to aerodynamic drag on the rotating stator, friction in various parts of the stator assembly, and a back EMF which is created as the stator coils rotate through the magnetic field of the motor. In order to maintain the relative angular speed of the spinning stator, power has to be supplied in some manner to the stator to overcome these losses.
At the same time, external forces applied to the rotor member, such as for example, gravity torque, or a torque produced by the probing force between the probe and a workpiece, act to slow the rotor down. This causes the control system to supply more power to the motor driving the rotor, to maintain the probing speed, and this additional power tends to then over-speed the freely spinning stator. To overcome this, a mechanism has to be provided which will apply a braking force to the spinning stator.
These problems are overcome in accordance with the present invention by providing an additional motor including a winding assembly and a magnet assembly, one of which is attached to the rotatable stator, and the other of which is attached to the fixed structure of the housing.
By this means, the additional motor provides a frictionless braking force on the free spinning stator which increases as the speed of rotation of the stator increases. Also when it is necessary to provide power to the stator to overcome losses, this is achieved by supplying current to the additional motor.
The reactionless behaviour in such a configuration is therefore limited to accelerations of the rotary drive mechanism. Reactions due to external probing etc are transmitted to the mountings of the mechanism, reactions due to rotary accelerations are not.